The current invention relates to a method and device for the mineral wool culture of plants.
Plants which are cultivated in a substrate of mineral wool, particularly rock wool and glass wool, receive either periodical or continuous applications of water and, where required, fertilizer, such that cultivation conditions for the plants are optimal.
The physical properties of the mineral wool used determine among other things the quantity of water that the mineral wool contains, how the delivery of water to the plant takes place, and how water is again absorbed. These physical properties can be derived and forecast from the characteristic moisture curve, the so-called pF curve, of the mineral wool. pF is the suction pressure (negative hydraulic pressure, usually expressed in cm water column) and indicates, as a function of the current moisture content of the mineral wool, the force with which water is held by the mineral wool, or in other words, how much force the plant must generate in order to extract water from the mineral wool (Tuinderij (Market Gardening) 1986, p. 54 and 55).
In order to limit the force which the plant must generate to absorb water, the suction pressure in the mineral wool should be kept as low as possible. This is, however, only possible to a very limited extent, because the water management is directly linked to the air management in the mineral wool. If mineral wool contains more water, then the air (oxygen) content decreases, and as a result the air supply to the plant roots is inhibited. For each mineral wool, there is therefore an optimal water/air ratio.
At the current time in the mineral wool culture of plants, water and any required fertilizer are added either continuously or periodically to the mineral wool, in order to compensate for the quantity of water which has been taken up (absorbed) by the plants, which has evaporated and which has drained out of the mineral wool. The water application is hereby geared to the water requirement of the plant or the crane section with plants. In other words the plant or crane section with the largest water requirement dictates the size of the water application. This implies that the remaining plants (with a differing water requirement) are cultivated under sub-optimal conditions.
At the current time the desired water application is determined with so-called starting trays. A number of plants are cultivated separately in measuring boxes in conditions that are as far as possible the same; the water consumption of the plants is derived from the amount of water supplied and the amount of drainage water. The water requirement of the plants in the starting tray is in this way approximated and adjusted to the current water requirement of all the plants present. This measured water requirement should be a measure for the actual water supply at that moment. The quantity of water supplied is however not the same as the determined, current water requirement. On the one hand because the current water requirement of each plant cannot be determined, the cultivation conditions for the whole culture deviate locally, and on the other hand because the quantity of water delivered via the drippers is not everywhere exactly the same. In fact, a more than sufficient quantity of water is given as a result, the excess being drained off. This entails extra material costs for the water consumed and the fertilizer absorbed in it, and also forms an increasingly greater load on the environment. Cultivation conditions are moreover sub-optimal.
The invention has for its object in the mineral wool culture of plants to cultivate all plants as far as possible in the same optimal conditions, irrespective of differences between their individual water requirements. In accordance with the invention each plant in fact individually determines the amount of water it requires. At the same time the water absorption has no influence whatever on the water/air management of the other plants, particularly neighbouring ones.
The invention is based on the concept that this aim can be achieved by adjusting the suction pressure in the mineral wool at a determined value using a capillary system with which water can also be supplied and/or discharged.
Should a particular plant (cultivated under a water/air management which can be set exactly) absorb water out of the mineral wool, a very small increase in the suction pressure will occur locally and temporarily. This is compensated for either directly or instantaneously by supplying water via the capillary system. It is very important to note here that this supply of water is exactly equal to the quantity of water taken up by the plant. The same applies with regard to compensation for the amount of water which evaporates and/or drains out of the mineral wool.
The suction pressure in the mineral wool is maintained at the constant, optimal value, so that neighbouring and other plants are in no way affected by water absorption by another plant. The air/water management thus remains substantially undisturbed.
It is noted that in open ground underground supply of water via a capillary line is known (Technical Information Reko Pearl; the grow tube, Charles H. Cordewener, October 1986).
Soil differs greatly in structure and texture from mineral wool. Soil has for example a density of 1.0-1.6 g/cm.sup.3 (mineral wool 0.04-0.1 g/cm.sup.3), and a porosity of 40-50% (mineral wool 90-95%). The most characteristic difference which is directly related to the water management is the difference in pore size distribution between soil and mineral wool. A pore size distribution of clay soil (P. Schachtschabel et al, Lehrbuch der Bodenkunde, 1982, p. 158) and mineral wool is for example shown in the table.
This means that soil has a pF curve differing widely from that of mineral wool. Soil can take up considerably less water, up to 40-50% by vol. (mineral wool up to 90-95% by vol.), but can retain water considerably better, suction pressure 125-20,000 cm water column (P. Schachtschabel et al, id.) (mineral wool 0-20 cm water column) (Chr. Blok, Grodan product information: Capillary dynamics (1985).
This means that soil is less quickly exhausted of water and as a result is highly safe for cultivating plants. An inherent disadvantage, however, is that the oxygen concentration of the soil can only be controlled by a comparatively great change in the suction pressure. On the other hand soil possesses a highly non-homogeneous mineral composition (clay, silt, sand) as a result of which it has a large exchanging capacity for minerals. This implies that the electrical conductivity (EC) can only be poorly controlled. Because of the irregular pore size, the washing characteristic is poor. The latter results in a high drainage percentage of &gt;20%.
Mineral wool on the other hand is highly porous and possesses a low density and distinct pore size distribution, while the structure and texture is substantially constant. This means that with only a relatively small change in the suction pressure (5 cm. water column) mineral wool easily takes up a lot of water, but can also lose it very easily. This involves great risks where control of the water/air management is concerned. When there is a water supply which differs from the current requirement during cultivation, this can lead to large variations in the moisture content of the mineral wool, which are of enormous significance for the cultivation of plants.
The French patent application 2,297,562 discloses a system of growing plants in an open container filled with finely divided sand. Water and optionally nutrients are fed to the container at a constant pressure of 0.1-0.2 kg/cm.sup.2. This pressure is chosen such that the amount of water added equals the average daily water consumption during the whole season, which quantity is 5 liters/m.sup.2 /day for tomatoes and 4 liters/m.sup.2 /day for lettuce. However, the water consumption differs significantly over the day and night period. At night the water consumption varies between 0-100 ml/m.sup.2 /hour while between 12:00 and 14:00 may be as high as 6000 ml/m.sup.2 /hour. The amount of water added is proportional to the sum of the water pressure in the inlet pipe and the suction pressure in the sand bed.
Accordingly in the French cultivation system the amount of water added during the night time is larger than the actual demand but during the day time is significantly less than the actual water demand. In other words during the night the sand bed is almost completely saturated with water whereas during the day time the water stored in the sand bed is used up to a large extent.
Such a system of confronting daily plants with an excess and a shortage of water cannot be used in a mineral wool culture because of the form of the pH curve, namely at a suction pressure of about 20 cm. water column the mineral wool hardly contains any water and the extraction of a relatively small amount of water will result in a tremendous increase of the negative pressure beyond the wilting point.
Although this conventional container filled with sand is provided with a discharge pipe, water is only discharged after harvesting the plants at the end of the growing season in order to wash the sand bed; the main object of this French application is to avoid any circulation of water and nutrients through the sand bed.